MAX6651EEE datasheet(5/25 Pages) MAXIM

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Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Detailed Description

The MAX6650/MAX6651 use an SMBus/I2C-Compatible

interface to regulate and monitor the speed of

5VDC/12VDC brush-less fans with built-in open-collec-

tor/drain tachometers. Regulating fan speed propor-

tionally with temperature saves power, increases fan

life, and reduces acoustic noise. Since fan speed is

proportional to the voltage across the fan, the

MAX6650/MAX6651 control the speed by regulating the

voltage on the low side of the fan with an external MOS-

FET or bipolar transistor.

The MAX6650/MAX6651 each contain two internal con-

trol loops. The first loop controls the voltage across the

fan. The internal digital-to-analog converter (DAC) sets

the reference voltage for an internal amplifier (Figure 1),

which then drives the gate of an external N-channel

MOSFET (or the base of a bipolar transistor) to regulate

the voltage on the low side of the fan. As the reference

voltage provided by the DAC changes, the feedback

amplifier automatically adjusts the feedback voltage,

which changes the voltage across the fan.

The second control loop consists of the internal digital

logic that controls the fan’s speed. The MAX6650/

MAX6651 control fan speed by forcing the tachometer

frequency to equal a reference frequency set by the

Fan-Speed Register, the prescaler, and the internal

oscillator (see the

Fan-Speed Register section). When

the tachometer frequency is too high, the value of the

DAC’s input register is increased by the regulator.

Once the DAC voltage increases, the analog control

loop forces the feedback voltage to rise, which reduces

the voltage across the fan. Since fan speed is propor-

tional to the voltage across the fan, the fan slows down.

2-Wire SMBus/I2C-Compatible

Digital Interface

From a software perspective, the MAX6650/MAX6651

appear as a set of byte-wide registers that contain

speed control, tachometer count, alarm conditions, or

configuration bits. These devices use a standard

SMBus/I2C-compatible 2-wire serial interface to access

the internal registers.

Pin Description

Maxim Integrated

MAX6650/MAX6651

FUNCTION

NAME

MAX6650

MAX6651

Tachometer Input. Used to close the loop around the tachometer.

TACH0

—

2, 3, 16

TACH2, TACH3,

TACH1

Tachometer Inputs. Used to monitor tachometers only.

Ground

GND

SDA

2-Wire Serial-Data Input/Output (open drain)

2-Wire Serial Clock Input

SCL

ADD

Slave Address Select Input (Table 1)

General-Purpose Input/Output (open drain). Configurable to act either as an out-

put or as an input (FULL ON or general purpose).

General-Purpose Input/Output (open drain). Configurable to act as a general

input/output line or an active-low ALERT output.

General-Purpose Input/Output (open drain). Configurable to act as a general

input/output line, an internal clock output, or an external clock input.

Output. Drives the external MOSFET or bipolar transistor.

+3.0V to +5.5V Power Supply

Feedback Input. Closes the loop around the external MOSFET or bipolar tran-

sistor.

VCC

OUT

GPIO2

GPIO0

GPIO1

—

7, 12

GPIO4, GPIO3

General-Purpose Input/Output (open drain)

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